It's the Disney malady. People raised on Disney expect an explicitly happy ending. I actually much prefer this kind of ending where you can think about what might happen after all this. I feel it's the final gift from the author to allow you to create the "head canon" or what you think how it may have ended. Give yourself permission to create the happiest ending from everything the author has set out for you until now. Or create for yourself a tragic, unfulfilled ending if you so choose. I personally think it's great that I can make it what I want it to be, after passively absorbing the story until now. And it's fun for the fans to compare and argue about which of their head canon makes the most sense, is the nicest, or more tragic, etc.
Sometimes, the author leaves it too much up in the air. And as with everything, sometimes they don't do it well. But I think with this one, he's provided a direction where it's most likely headed, and most head canon will flow to that direction. But there's still enough for it to go the other way if those fans want it that way. In many ways, this is perfect. Because even the happiest ending can't satisfy all the fans because it's static and the fans' tastes are varied and even malleable.
All it takes is you giving yourself permission to create the ending you want. Use your imagination. Japanese fans are used to it.